1. Field of the Invention
This invention relates to the field of integrated circuits. More particularly, this invention relates to integrated circuits having virtual power rails connected to a main power rail via a switch block.
2. Description of the Prior Art
It is known to provide integrated circuits including power rails connected via switch blocks to virtual power rails. The logic blocks within the integrated circuit then draw their power from the virtual power rails. The switch blocks, which are typically high threshold voltage header and footer transistors, can be used to isolate the virtual power rail from the main power rail and accordingly isolate the logic blocks from the power supply. This is useful in reducing power consumption of the integrated circuit, e.g. by reducing the static leakage current therethrough.
While the above approach is successful, it suffers from the problem that state (e.g. data values, instruction values, configuration etc) within the logic blocks is lost when they are isolated from the power supply. Furthermore, it can take a disadvantageously long period of time to restore this state and recommence processing using the logic blocks when it is desired to switch out of the low power mode. One way of at least partially addressing these problems is to provide balloon latches within the logic blocks to store relevant state signal values using circuits which do remain continuously powered (and are typically formed with high voltage threshold transistors having low leakage currents) so that the signal state values can be restored into the logic block when required and then processing recommenced. A problem with this approach is the overhead associated with the circuit area of the balloon latches, the control thereof and the time taken to restore the signal values from the balloon latches into the logic blocks.